Greasiest fop about to hold a significant birthday ceremony? (4,2,7)
I believe the answer is:
rite of passage
'significant birthday ceremony?' is the definition.
(a milestone birthday is a rite of passage)
'greasiest fop about to hold a' is the wordplay.
'about' indicates anagramming the letters.
'to hold' indicates putting letters inside.
'greasiest'+'fop'='greasiestfop'
'greasiestfop' anagrammed gives 'riteofpassge'.
'riteofpassge' going around 'a' is 'RITE OF PASSAGE'.
(Other definitions for rite of passage that I've seen before include "Significant event during a transitional period in life" , "Very important life event" , "The Wedding, perhaps?" , "Formative experience" , "Ceremony marking a change of status" .)